Per-tone control of a cellular OFDM system provides a scheme where the base station allocates an OFDM tone to a particular user who has the best channel conditions for that tone, so as to maximize the average throughput of the system. In contrast to a per-tone rate control of a cellular OFDM, a single-rate scheme allocates all of the OFDM tones to a particular user who has the best channel quality indicator (CQI) and communication is established using a fixed modulation and coding scheme (MCS) across all the tones. The per-tone rate control scheme achieves the maximum multiplexing gain possible whereas the single-rate scheme achieves the maximum diversity gain possible.
The feedback requirement for per-tone rate control on the uplink is directly proportional to the number of OFDM tones, while on the downlink, the broadcast requirement is proportional to the number of users serviced in the cellular system and the number of modulation and coding schemes in the MCS set. Thus, it is imperative that efficient feedback and scheduling schemes for a high load, multi-carrier system with a large number of data carriers be employed.
In a multi-user, multi-carrier cellular OFDM system employing per-tone rate control, the base station has to know the amplitude of the channel frequency response of the different users, so that it can assign each OFDM tone to the user with the best channel conditions for that tone. One way to effect this feedback is to communicate the channel frequency response across the data tones from each user to the base station. The feedback requirement with this scheme is directly proportional to the number of data tones, and is operationally prohibitive in a system with a large number of data carriers.
A simple feedback scheme may be developed if realistic physical channels can be modeled accurately with a few uncorrelated filter taps. The user then feeds back the quantized channel taps to the base station. The base station estimates the amplitude of the channel frequency response from the channel taps that are fed back. This scheme requires a feedback rate proportional to the number of filter taps modeling the channel between the base station and the user. The disadvantage with this scheme is that, since channel coefficients change randomly from one time instantiation to another, the effective feedback rate is also proportional to the rate of change of channel coefficients, which is usually high for a fading wireless channel.
The single-rate scheme minimizes the feedback on the downlink when compared with a per-tone rate control scheme, since only one user and MCS information has to be transmitted for all the OFDM tones. Here, the downlink feedback is immensely reduced as the base station has to inform the users of only one allocated user and MCS information. However, without further information, the uplink feedback requirement remains the same, as each user has to communicate the CQI for all the data tones.
Accordingly, what is needed in the art is an enhanced way to overcome these disadvantages and limitations.